System for digitally transmitting and displaying texts on television screen

ABSTRACT

In the disclosed teletext system, the transmitter station broadcasts data cks data from several channels, the data packs being time multiplexed. Each data pack comprises a prefix signal for causing a teletext receiver either to accept the data packs of a selected channel or to reject data packs from other channels. In addition to the prefix, and possibly page flags followed by page numbers and row flags followed by row numbers, the sequence of eight-bit words, transmitted in a pack, may comprise escape eight-bit words which are followed by type identity eight-bit words, themselves followed by a sequence of characters to be displayed in the same type the identity of which is determined by the preceding type identity eight-bit words. Each teletext receiver comprises, a first conventional random access memory storing the coded characters, a second random access memory storing the type identity eight-bit words at the same addresses as those of the characters belonging to the sequence in the first random access memory. A read operation from the first random access memory, for display purpose, is performed at the same time as the read operation from the second random access memory. The output signals of the second memory select a character generator which includes a permanent memory which reads out the shape of those characters which read out from the first random access memory.

The present invention relates to improvements to systems for digitallytransmitting texts and displaying them on a television screen. In thefollowing description such systems will be called "teletext systems".More particularly this invention relates to improvements to the teletextsystem that has been described in the U.S. patent application Ser. No.830,165, filed Sept. 2, 1977, (corresponding to the French PatentApplication No. 76 27212 filed on Sept. 6, 1976 in the joint names, Mr.A. Poignet, Miss Le Marouille and Mr. Fournier).

In the teletext system described in that patent application, a signalsource broadcasts or transmits data in the form of "data packs." Thosedata packs may be delivered from a plurality of channels. The set ofdata packs regarding a channel constitutes a "magazine" made of aplurality of "pages". Data packs from various channels can be encodedand multiplexed as described in the U.S. patent application Ser. No.689,493 filed on June 3, 1976 in the name of Y. M. Guinet et al andentitled "One way data transmission system". Data packs belonging to amagazine page are not time multiplexed with data packs of other pages ofthe same magazine. In a page, data begins with a page flag followed bydata indicating the page number, and ends with the next page flag. Ineach page of data are grouped in datarows. The word "row" thatcorresponds to a character line in a page is utilized to avoid anyinterference with television scanning lines. In each row, data arepreceded by a row flag followed by the row number, and followed by thenext row flag.

In every system receiver a decoding key may be used by a subscriberviewer which enables him to select the data packs relating to thedesired channel, data packs belonging to other channels being cancelled.Thus, a decoding key makes it possible to receive all the data packs ofa magazine that the source is periodically broadcasting. Moreover theteletext system receiver comprises a keyboard on which the subscribermay dial the number of a desired page within a magazine so that onlythose data which belong to the selected page are trasmitted to theteletext system receiver character generator.

The present invention relates to the selection of a magazine by means ofa decoding key. It does not relate to the selection of a page by meansof a keyboard. But it relates to an arrangement of transmitted signals.Means are provided in each teletext system data receiver for possiblyvarying the appearance of texts displayed on television screens so as todepart from the current display appearances. For instance, thosevariations may affect the character color, background color, flashing onor off for certain characters, changes in character height and/or width,etc.

A character generator for displaying characters on a television receivercathode ray tube screen comprises a permanent memory associated with arandom access memory. The permanent memory stores the shape informationfor every character which can be displayed. The random access memorystores the character codes, including the spacings or blanks betweenwords forming the text to be displayed. In the random access memory, thecharacter codes are stored at addresses which correspond to thegeographic positions of the associated characters in the page to bedisplayed. The display operation includes a sequential read-out of therows of character from the permanent memory for causing, in a knownmanner, the scanning read out of the character shape of the charactercorresponding to the read character code. Should a character occupy tentelevision scanning lines, each read out operation for a row ofcharacter codes, from the random access memory, results in tensuccessive scannings of the character shapes from the permanent memory.Obviously, scannings are, in turn controlled by clocks which arecontrolled from the display tube line synchronization. The signalsresulting from scanning the shape of a character are delivered inparallel, then converted into a serial mode through a shift registerwherein the shift signal frequency is controlled by a clock that is, inturn, controlled from the display tube line synchronization. The outputof the shift register is sampled at another frequency which iscontrolled by another clock, and the shift register output signal istransmitted toward the tube as a luminance signal.

Furthermore, in character transmission systems wherein each character isrepresented by an octet, i.e. an eight-bit word, the ASCII code iscurrently in use. Among the eight bits of a word, one bit is normallyassigned to parity check. The 7 other bit provide 128 characterpossibilities. Among those 128 possibilities some are assigned tocharacter codes and others to function codes. In addition, an escapeeight-bit word makes it possible in practice to double thosepossibilities in currently changing the meaning of codes following anescape word.

A purpose of the present invention is to provide a teletext systemwherein the display characters may be of various types that aredifferent each from the other in the shape, color, size, background, andso on, each type of displayed character being identified by means of oneor several eight-bit words corresponding to the character type identity.

According to another feature of the present invention, there is provideda system for digitally transmitting and displaying texts on a televisionscreen wherein the sequence of the transmitted eight-bit words comprises(in addition to prefixes, page flags followed by page numbers, and rowflags possibly followed by row numbers) escape eight-bit words followedby type identity eight-bit words. These signals are followed by asequence of characters to be displayed in the same type, the identity ofwhich is determined by the preceding type identity eight-bit words. Eachsystem receiver includes a first conventional random access memory forstoring the coded characters, and a second random access memory forstoring the type identity eight-bit words at the same addresses as theaddresses of the characters belonging to the sequence in the firstrandom access memory. A read operation is taken from the first accessmemory, for display purpose at the same time as the read operation fromthe second random access memory whose output signal selects a charactergenerator permanent memory, from which the shape of characters read outfrom the first random access memory is to be read out, and/or controlsthe luminance and/or chrominance in the display tube at the time whenthe characters are displayed on the television tube.

According to another feature of this invention, the type identityeight-bit words following an escape eight-bit word comprise an identitycomponent eight-bit word followed by an escape eight-bit word percharacter identity component, save the last identity component eight-bitword that is directly followed by the first character eight-bit word ofa sequence of characters which the type is assigned to. The receiveddata is applied to a switching circuit for delivering, on the one hand,the escape eight-bit words followed by a type identity eight-bit wordsserially to a buffer memory and, on the other hand, character eight-bitwords which are or escape eight-bit words not followed by type identityeight-bit words. This data is applied to the input of the first randomaccess memory. A buffer memory output is connected to the second randomaccess memory input for delivering its contents, save escape eight-bitwords, to the second random access memory each time a characterbelonging to the sequence is written into the first random accessmemory. The contents of the buffer are erased when the next escapeeight-bit word appears followed by a type identity or type componentidentity eight-bit word.

The above mentioned features of the present invention, as well as otherfeatures thereof will appear more clearly in the following descriptionof an embodiment, the description being made in conjunction withaccompanying drawings wherein:

FIG. 1 is a block diagram of the teletext system receiver described inthe first above mentioned U.S. patent application,

FIG. 2 is a block diagram of memory circuits and character generatorwhich are usable, according to this invention, in the teletext systemshown in FIG. 1, and

FIG. 3 is a schematic diagram of a mixer circuit utilized in thereceiver shown in FIG. 2.

The receiver terminal (FIG. 6 herein) shown in FIG. 2 in the firstmentioned U.S. patent application, the description of which mayconsidered in the present description. The receiver shown in FIG. 1comprises, as a conventional television receiver, areception-demodulation circuit 10 delivering, on the one hand, audiosignals to a loud-speaker 11 and, on the other hand, video signals to acolor decoder and scanning generator 12. Color signals B1 (blue), V1(green) and R1 (red) as well as luminance signal from 12 are nottransmitted directly to tube 13, contrary to what occurs in aconventional television receiver these signals are delivered through avideo switch 14 that also performs the adaptation of the tubevoltage/light characteristic to the control of the synthesized signals.

Reception terminal shown in FIG. 1 further comprises a circuit 15 forseparating data from the video signal, whose input is connected fromvideo output of circuit 10 and output is connected to a first selectionblock 16, which is similar to the terminal equipment described in U.S.patent application No. 689,493. The output of block 16 is connectedthrough a link J to a page selection block and data decoder 17 whoseoutput is connected to the input of a page memory 18. A viewer kayboard19 has its output suitably connected to control inputs of blocks 16 and17. The output of memory 18 is connected to the input of a charactergenerator 20. The signal outputs of character generator 20 are connectedto color inputs R2 (red), V2 (green) and B2 (blue) of video switch 14 aswell as to a luminance input 21. A control output 22 of keyboard 19 isalso connected to a control input of video switch 14.

It is to be noted that link J is preferably of the type described in theFrench Pat. No. 2,268,308. It makes it possible an asynchronousoperation from selector 16 to page selector 17. Furthermore, in analternative embodiment, the input of link J may be connected from acable teletext distribution network, for instance utilizing the publictelephone network.

In circuit 15, a stable oscillator, not shown, regenerates the bit ratefrequency. Then, processing circuits treat the bits arranged in 8-bitwords or octets as described in U.S. patent application No. 689,493,particularly regarding validation by a key for receiving the desiredchannel, as described in FIG. 6 of the U.S. patent application.

Assuming the validation is positive, the received bits are sent to block17 through link J. In practice, in the described embodiment, a channelis equivalent to a magazine. Block 17 includes means for recognizing andcomparing a page flag, 8-bit words. Each time that the recognition ispositive, the immediately following page number 8-bit word is thedesired page number which was dialled by the viewer on keyboard 19. Whensuch a comparison is negative, received 8-bit words are destroyed aswell as the following ones, up to the next recognition of a page flag.When such a comparison is positive, the received 8-bit words, up to thenext page flag, are transmitted to memory 18 after having been arrangedin rows by appropriate means, as described in the first mentioned U.S.patent application. It is simply remembered that each row is preceded bya row flag, followed by a 8-bit word indicating the height of the row onthe page and defining one address for the characters to be stored inmemory 18, the other address for each character being defined by theposition of the character 8-bit word, in the row.

In the described embodiment according to the present invention, pagememory block 18 and character generator 20 are designed as shown in FIG.2. In practice, memory 18 comprises two random access memories or RAMmemories 23 and 24, as well as a switching circuit 25 and a buffermemory 26. The input of switching circuit 25 is connected, via line 27,from the output of circuit 17, FIG. 1 that delivers character 8-bitwords and row function 8-bit words. Row addressing inputs in memories 23and 24 are parallel connected, via line 28, from output of circuit 17which delivers the row numbers. Switching circuit 25 has an output 29connected to the data input of memory 23 and an output 30 connected tothe input of buffer memory 26. The output of buffer memory 26 isconnected to the data input of memory 24.

Data which is applied to the input of switching circuit 25, is receivedvia line 27, and may be grouped in the following families: first, in thesimplest case, an alphanumeric character 8-bit word; then an escape8-bit word plus either a character 8-bit word or a conventional function8-bit word; and finally an escape 8-bit-word plus a character typeidentity 8-bit word. Switching circuit 25 includes detection means forrecognizing the third family of data (escape 8-bit word plus charactertype identity 8-bit word). Those detection means may possibly be simplecomparators, which makes it possible to send them to output 30 while thefirst and second family of data are sent to output 29.

By way of example, some types of identity 8-bits words will now beillustrated. A 8-bit word F1 may be provided wherein the five first bitscarry, when they are in the O-condition, (i.e. have no information).

    ______________________________________                                        1st  2nd     3rd     4th    5th  6th   7th   8th                              bit  bit     bit     bit    bit  bit   bit   bit                              ______________________________________                                        Red  Green   Blue    Graphic                                                                              1    0     0     X                                ______________________________________                                    

Thus, a 8-bit word 1000100x indicates that the sequence of thecharacters following it must be displayed in red on the tube screen. Byway of example, with a 8-bit word 0110100X, it indicates that thesequence must be displayed in cyan. When the 5th bit is 1, it indicatesthat the 8-bit word is a word F1; when 6th and 7th bits are 0, switchingcircuit 25 can detect the function 8-bit words. The meaning of the 4thbit will be described in the following specification, the 8th bit is aparity bit.

In an same manner, a 8-bit word F2 may be provided wherein the bits havethe following meanings:

    ______________________________________                                        1st   2nd     3rd      4th    5th  6th  7th  8th                              bit   bit     bit      bit    bit  bit  bit  bit                              ______________________________________                                        Height                                                                              Width   Ground   Flashing                                                                             0    0    0    X                                ______________________________________                                    

When the first bit is 1, it indicates that the character height is to bedoubled; when the 2nd bit is 1, it indicates that the character width isto be doubled; when the 3rd bit is 1, it indicates that the back groundonto which the character is to be displayed must have its colorreversed; when the 4th bit is 1, it indicates that the character must beflashed on and off. Thus, an 8-bit word 110100X, indicates that heightand width of the characters must be doubled and that they must beflashed on and off. The 5th bit, being 0, indicates that the 8-bit wordis a word F2.

Now it is possible to understand that, when, in the sequence of an 8-bitwords applied from switching circuit 25 to line 27, there is thefollowing series: θ F1 θ F1 a b c . . . , switching circuit 25 willwrite the four words θ F1 θ F1 into buffer memory 26; then, the words ab c . . . , are written into memory 23. The writing of the firstcharacter word a into memory 23 is detected in circuit 31 which theninitiates writing at the same address into memory 24 of word of thewords F1 and F2 from buffer memory 26 (words θ being not transmittedfrom buffery memory 26 to memory 24). Circuit 31 is connected by controlline 32 to memory 26. Buffer memory 26 is connected to RAM memory 24 byline 33.

The read out from buffer memory 26 into random access memory 24 does notalter the contents of memory 26. Thus, when writing b into random accessmemory 23, F1 and F2 are written at the same address as b into memory23. As a conclusion, at corresponding addresses, character words andidentity words are written into memory 24. Contents of memory 26 areerased at the next writing into that buffer memory 24 of an escape wordfollowed by an identity word.

In addition of function words F1 and F2, other function words may alsobe provided, as for instance F3 making it possible to change from afirst character shape to another character shape, together 5th and 7thbits, and then indicate an identity word which is detected in switchingcircuit 25, 6th bit being 0 and 8th bit still being a parity bit.

Character generator 20, shown in FIG. 2, comprises four permanentmemories 34-37, a switch 38, a mixing circuit 39 and a monostablecircuit 40. Each permanent memory 34-37 is associated respectively witha shift register 41-44 designed for delivering serially, the bitsconventionally read in parallel from a character generator into apermanent memory. It is reminded that the scanning of character shape isperformed in memories 34-37, line per line, each line basicallycorresponding to a television picture line. A clock 45 is also provided,which controls read operations from the memories in accordance withtelevision picture line synchronization signals. In FIG. 2, some inputsS are also indicated, which mean that television line synchronizationsignals are also used for reading the concerned memories or controllingrepetitive operations; however, to simplify the drawing, all the inputsS are not indicated.

Switch 38 is provided for selecting the one of the registers 41-44 thatis through be to its output 46 connected to mixing circuit 39; it alsodetermines the read-out operation rate, or in other words the samplingrate of register outputs.

The read-out output of RAM memory 23 is parallel connected to read-outaddress inputs of permanent memories 34-37. Parallel read-out, outputsof memories 34-37 are respectively connected to parallel inputs ofregisters 41-44.

The read-out output of RAM memory 24 has as there are many wires assignificant bits in function words F1, F2 and F3. Thus indicated arewires R, G, B respectively corresponding to the first 3 bits of F1, wireH corresponding to the third bit of F2 and wire C corresponding to thefourth bit of F2. In addition, there is a line M comprising four wirescorresponding to the significant bits, of F3, which are in the describedexample assumed to be two bits since four permanent memories areprovided.

Wire H is connected to a control input of clock 45. When wire H is at alow potential, clock 45 operates at a normal rate, i.e. for each pictureline RAM memory 23 is read as well as a character shape line stored inthe permanent memories 34-37, that line corresponding to the addressread out from memory 23. When wire H is at a high potential, the clockrate of clock 45 is divided by two. Clock 45 designates, after counting,the address of the line to be read in the character shapes of thepermanent memories. When wire H is at a high potential, each charactershape line is successively read twice at the picture line frequency.Thus the height of the displayed character occupies twice the normalnumber of lines on the tube screen. Its height is doubled.

Wire L included two wires, one wire L1 controlling the bit shift rate inregisters 41-44, the other wire L2 controlling the horizontal addressprogress rate in RAM memory 23, via input L2. Obviously those two ratesare also related to the television picture line synchronization. Whenwire L is at a low potential, the shift rate and sampling rate arenormal. When wire L is at a high potential, the shift rate and samplingrate are both divided by two so that the same bit stays twice the normallength of time in the output cell of each shift register and thereforecan be sampled twice. Thus a character point information can be sent tothe display tube in the picture line. Accordingly the character width isdoubled. As a character then occupies a wider place in the displayedrow, it is necessary, through wire L2, to reduce by two the progress ofthe character words in the corresponding row in 51.

The four wires of line M make it possible to select the permanent memorywhose output is utilized among memories 34-37. Indeed it appears that ateach read-out operation from memory 23, the four memories 34-37 areparallel read out. However, switch 38 allows only one connection to bemade between circuits 34-37 and the mixer circuit 39. The four permanentmemories 34-37 respectively store four alphabets. Thus, it appears thattwo bits of words F3, in a simple manner, change the language dependingon the texts to be displayed.

When the potential on wire C is low, monostable circuit 40 is at rest.When the potential on wire C is high, monostable circuit 40 is triggeredon. The monostable circuit rate may be about 1 Hz and is used asdescribed hereabove for flashing characters on and off.

Reference will now be made to FIG. 3 for describing the purposes ofwires R, V (or G), B and F. FIG. 3 still shows wire 46 connecting theoutput of switch 38 (FIG. 2) to mixer circuit 39, as well as output wire47 from monostable circuit 40 (FIG. 2). First it is reminded that wire46 carries digital signals relating to the luminance information. Ateach character shape line scanning in a permanent memory, and after theparallel-serially conversion in the corresponding shift register, thesignal transmitted from switching circuit 38 over wire 46 is shaped asindicated by the wave form l, FIG. 3. When signal l is high, a spot islit on the tube; when signal l is low, the background does not light.Wire 46 is connected to input of a switchable inverter circuit 48 whoseoutput is connected to a switch 49. The signal output of switch 49 isconnected, on the one hand, to output 21 through a shaping circuit 50and, on the other hand, and in parallel, to the first inputs of threeAND gates 51, 52 and 53. The second input of gate 51 is connected fromwire R, the second input of gate 52 is connected from wire V (or G), andsecond input of gate 53 is connected from wire B. The output of gate 51is connected to wire R2 through a shaping circuit 54, the output of gate52 is connected to wire V2 through a shaping circuit 55, and the outputof gate 53 is connected to wire B2 through a shaping circuit 56. Thecontrol input of switchable inverter circuit 48 is connected to A wire Fwhile the control input of switch 49 is connected to A wire 47.

Assuming that wires F and 47 are at rest in the 0-condition or lowpotential, signal l from wire 46 is applied without change to switchingcircuit 50 and gates 51-53. From output 21 of circuit 50, the outputsignal is processed in the switching circuit 14, FIG. 1, shown in detailin FIG. 3. In addition, assuming that wire R (or G) is in the1-condition or high potential, conditions 1 of signal l are transmittedthrough gate 51 and applied to shaping circuit 54 before being processedin switching circuit 14, FIG. 1. Thus it appears that the redchrominance signal is similar to the luminance signal that causes thecharacter to be displayed in red. When wires R and V (or G) are high,conditions 1 of l are transmitted and wires R2 and V2 become high, thecolor mix being performed in the tube 13 after processing in switchingcircuit 14. Thus, it characters may be displayed in white or one of sixpredetermined colors by using the first three bits of word F1.

When wire F is in the 1-condition or high potential, the output signalfrom switchable inverter 48 is reversed with respect to l, i.e. l. Suchan output signal is still processed in gates 51-53, as signal l, butcontrary to the previous described case, only the parts outside of thecharacters corresponding to conditions 0 of 1 appear displayed in color.

When wire 47 is periodically turned to the 1-condition, transmission ofsignal l is interrupted through circuit 49 during those 1-conditions. Asa result the luminance signal is accordingly interrupted which causesthe character to flash on and off on the display.

Shaping circuit 50 is also utilized for adding the digitalsynchronization signal to the luminance signal, as indicated by S.

Also to be noted that, with no type identity information, outputs RVB ofrandom access memory 24 (FIG. 2) are at a high potential or in1-condition to display white characters on a black background.

Read out operations are obviously performed at the same time in RAMmemories 23 and 24 so as to read characters from permanent memories atthe same time as conditions of output wires of 24 are controlled byidentity bits in words F1, F2 and F3.

With respect to the bit corresponding to "Graphic" in word F1, it mustbe understood that, for instance close to permanent memory 34, a wiredmemory stores six-point rectangles, thus making it possible bydisplaying them side by side to produce graphics on the screen. Inpractice, the output of RAM memory 24 has a wire corresponding to the4th bit of F1 which permits a selection the wired memory instead of thepermanent memory 34 in a same manner as the selection controlled by wireM. Graphic rectangle codes are provided in the ACSII code.

In an alternative of the operation of the teletext system according tothis invention, it is preferred to begin each row with a type identityword. The character type may possibly be changed within the row, but itreturns to the initial type at the end of each row. Thus the display isresynchronized at the beginning of each row.

While the principles of the present invention have hereabove beendescribed in relation with a specific embodiment, it must be clearlyunderstood that the description has only been made by way of example andnot as a limitation to the scope of this invention.

What is claimed is:
 1. A receiver for a teletext system for digitallytransmitting and displaying texts on a television screen, wherein atransmitter station broadcasts signals representing said text, saidsignals being in the form of data packs, sent over several channels, thedata packs from different channels being randomly time multiplexed, eachpack comprising a prefix signal for enabling a receiver to accept thedata packs received over at least a selected channel and to reject datapacks received over other channels, each of said data packs includingthe transmission in sequence of eight-bit words comprising, in additionto said prefix, page flags followed by page numbers and row flagsfollowed by row numbers, escape eight-bit words followed by typeidentity eight-bit words, type identity eight-bit words, followed by asequence of coded words identifying characters which are to be displayedin the same type which is identified by the identity words as determinedby the preceding type identity words, said words receiver comprisingfirst random access memory means for storing the coded character wordsat certain addresses, second random access memory means for storing thetype identity words at the same certain addresses as the addresses ofcorresponding character words stored in the first random access memory,means for reading the words stored in the first random access memorymeans and at the same time as the read out if the words stored in thesecond random access memory, thereby simultaneously selecting acharacter and a type identity, and permanent memory means responsive tothe read out of said second random access memory means for giving theshape of the characters read out from the first random access memory. 2.A teletext receiver for the system according to claim 5, and meansresponsive to output signals from the second random access memory meansfor controlling the luminance or chrominance in the display tube wherethe characters are displayed on the television tube.
 3. A teletextreceiver for the system according to claim 5, wherein the type identityeight-bit words following an escape eight-bit word comprise an identitycomponent eight-bit word followed by an escape eight-bit word percharacter identity component, except for the last identity componenteight-bit word that is directly followed by the first charactereight-bit word of a sequence of characters to which the type is assignedand buffer memory means, switching circuit means for applying thereceived data to deliver the escape eight-bit words followed by typeidentity eight-bit words serially to said buffer memory means, saidswitching circuit means also delivering character eight-bit words orescape eight-bit words not followed by type identity eight-bit words tothe input of the first random access memory means, the buffer memorymeans output being connected to said second random access memory inputfor delivering its contents, except for the escape eight-bit words, tosaid second random access memory means each time a character belongingto said sequence is written into the first random access memory means,the contents of the buffer memory means being erased when the nextescape eight-bit word followed by a type identity or type componentidentity eight-bit word is received.
 4. A teletext receiver for thesystem according to claim 2 wherein the type identity eight-bit wordsfollowing an escape eight-bit word comprises an identity componenteight-bit word followed by an escape eight-bit word per characteridentity component, except for the last identity component eight-bitword that is directly followed by the first character eight-bit word ofa sequence of characters to which the type is assigned, and buffermemory means, switching means for applying the received data escapeeight-bit words followed by type identity eight-bit words serially tosaid buffer memory means and character eight-bit words or escapeeight-bit words not followed by type identity eight-bit words to theinput of the first random access memory means, said buffer memory meansoutput being connected to said second random access memory means inputand delivering its contents, except for escape eight-bit words, to saidsecond random access memory means each time a character belonging tosaid sequence is written into the first random access memory means, thecontents of the buffer memoring means being erased when the next escapeeight-bit word followed by a type or type component identity eight-bitword is received.